The invention pertains to the field of meters for measuring the fluid permeability of solids. More particularly, the invention pertains to apparatus for holding samples for permeability measurement.
The permeameter is an instrument which is well known to the art. A permeameter determines the permeability of a sample by applying a fluid, such as air, under pressure to one side of the sample and measuring the rate at which the fluid flows through the sample. The permeability is then derived as a function of the flow rate, sample size, and pressure differential across the sample.
This method of determining the permeability assumes that the only route for fluid flow is through the sample. Obviously, any fluid flow around the sample instead of through it, or leakage from the system, will adversly effect the accuracy of the results obtained. Thus, the accuracy of the permeameter is dependent upon the method used for holding the sample. Unfortunately, many prior art permeameters were incapable of preventing fluid flow outside of the desired route. A significant portion of the fluid leaked past the sample.
A common method of holding samples used by prior art permeameters is shown in FIG. 4. The sample (30) is compressed between two plates (28) with O-ring seals. The fluid (34) is introduced through the top plate and flows out the bottom, in theory. In practice, however, the fluid is prone to bypassing the sample through its sides, as shown by arrows (30). Also, without careful machining of the top and bottom surfaces of the sample, there can be leakage between the O-rings and the sample.
Other permeameters (FIG. 5) require the sample (30) to be encased in a rigid tube or cylinder (32). Again, the sample must be accurately machined, this time on the curved surface of a cylinder, which is obviously difficult. Gaps (33) are formed between the sample and the cylinder and accuracy is lost. For an example of this system see U.S. Pat. No. 2,686,425, issued to H. W. Dietert in 1954.
Still other permeameters have used a system of encasing the sample in a tapered rubber plug and forcing the plug into a tapered hole. Some examples of this may be found in U.S. Pat. No. 2,521,079 [1950] and U.S. Pat. No. 2,633,015 [1953], both issued to W. L. Morris, as well as in Welge, U.S. Pat. No. 2,465,948 [1948] and Davies, U.S. Pat. No. 3,181,346 [1965]. Although an improvement over the earlier rigid cylinder systems, this method is limited by the ability of the thick rubber plug to conform to irregularities on the sample surface. The plug will compress, to an extent, when forced into the tapered hole, but the compression is limited and uneven, again due to the thickness of the rubber. The thinner end will compress more easily and grip the sample more tightly than the thicker end, which will tend to lead to gaps between the sample and the plug, toward the thicker end. Alternatively, the sample could be overtightened on the thinner end and crushed in an attempt to close the gaps on the thicker end.
Finally, some permeameters have attempted to solve the leakage problem by surrounding the sample in a mercury bath (see TenBrink, 2,724,963 [1955] and Hertzog, U.S. Pat. No. 2,737,804 [1956]. This introduces the use of possibly toxic mercury into the process, and increases the expense and complexity.
It is, therefore, an object of the invention to provide a permeameter which will measure permeability accurately with a minimum of error introduced by leakage around the sample.
It is a further object of the invention to provide a sample holder for permeameter which will accept samples in a variety of sizes, without extensive modification.
It is a still further object of the invention to provide a sample holder for permeameter which will accept samples without extensive machining of the surface, without major error being induced by leakage over an irregular surface.
It is another object of the invention to provide a sample holder for permeameters which meets the above objects and is simple to use and inexpensive to produce, and which does not require the use of liquids or metals surrounding the sample to provide an accurate seal.
Other objects of the invention will become obvious upon reference to the disclosure below.